It's probably a great surprise to you, but one of the bigger challenges for growing cabbage is a creature called a cabbage moth. And cabbage moths arrive to the cabbage by (of all things) flying.
So this "fabric" (it's non-woven) is called Reemay. "This polyester fabric allows 75% light transmission. Provides frost protection down to 30°F"
And it keeps cabbage moths away.
I'm told that another option is to visit the fabric store and pick up some tulle. (Noun: A soft, fine silk, cotton, or nylon material like net, used for making veils and dresses.) So the Bridal store becomes the Garden store? Hmm.